The present invention generally relates to processes used in the removal of pollutants from gases, such as from utility and industrial flue gases. More particularly, this invention is directed to a modular circulating dry scrubber system.
Various processes are used to remove substances such as acidic constituents and particulate matter from combustion or flue gases produced by utility and industrial plants. For example, acidic gases, including sulfur dioxide (SO2), hydrogen chloride (HCl) and hydrogen fluoride (HF), are known to be hazardous to the environment, and as a result their emission into the atmosphere is closely regulated by clean air statutes. Common processes for treating combustion or flue gas include wet or dry flue gas desulfurization (FGD) systems, for example, limestone wet FGD, lime-based spray drying absorption (SDA), and circulating dry scrubber (CDS) systems (also referred to as circulating fluid-bed (CFB) dry scrubbers).
CDS systems use dry sorbent circulation in a reactor (absorber) to achieve capture of pollutants, such as SO2, SO3, HCl, and HF. In typical CDS systems, a dry sorbent (reagent) such as hydrated lime is humidified with water and injected into the bottom of a reactor concurrently with a flue gas to be scrubbed. As it enters the reactor, the flue gas is directed in an upward direction and accelerated through a Venturi before entering a circulating fluidized bed (CFB) portion of the reactor. Within the fluidized bed, contact between the flue gas and humid sorbent results in a high removal efficiency of pollutants from the flue gas. Pollutants such as SO2, SO3, HCl and HF can be removed with hydrated lime to form compounds such as CaSO4, CaSO3, CaCl2, CaF2, etc. The resulting cleaned (scrubbed) flue gas along with the fluid bed solids (particulates including sorbent, sorbent with absorbed pollutants, and fly-ash) exit the top of the reactor and enter a particulate collector (control device), where the fluid bed solids are removed from the scrubbed flue gas stream. The scrubbed flue gas is then typically directed to either a booster or induced draft (ID) fan before being discharged to a stack. Conventionally, a large portion of the sorbent within the fluid bed solids remains unreacted and is recycled back into the reactor as a dry powder to maintain the circulating fluidized bed in the reactor.
Conventionally, a CDS system would be configured to handle the total gas volume from a boiler through one or more venturies in one or more reactors. A particulate control device, such as a multi-compartment fabric filter (FF) or, in some applications, an electrostatic precipitator (ESP), is generally located downstream of each reactor at a significant elevation above grade so that a large quantity of collected fluid bed solids can be recirculated back to the fluidized bed, for example, via air-slides. At certain levels of reduced load, CDS systems further typically require recycling a portion of the scrubbed flue gas to the reactor inlet to maintain the fluidized bed within the reactor. The recirculated scrubbed flue gas may be routed through a gas recirculation duct or similar component from the discharge of the booster or induced draft fan. This recirculated gas is typically at a lower temperature than the incoming flue gas and can adversely affect the overall performance of the air pollution control system. Additionally, the high concentrations of particulate (sorbent and fly-ash) exiting CDS systems have been known to create operational difficulties with inlet dampers upstream of particulate control devices.
In view of the above, it can be appreciated that improvements to CDS systems are continuously sought, a particular example of which would be an improved CDS system capable of operating without relying on the use of recirculated scrubbed flue gas to maintain a fluidized bed within the CDS reactor.